Corrosion-resistant metallic article and method of making the same



Patented Apr. 8, 1947 UNITED STATES- PATENT OFFICE CORROSION-RESISTANT METALLIC ARTI- CLE AND IVIETHOD OF MAKING THE SAME John S. Thompson and BobertR. Tamer, De-

troit, Mlch., assignors to Parker Rust Proof Company, Detroit, Mich. 1

No Drawing. Application June 3, 1940, Serial No. 338,582. In Canada May 22, 1939 2 Claims. 1

.paring metals for paint, to treat the metal with solutions of phosphates or oxalates by means of which phosphate or oxalate coatings are produced. These coatings result from an attack on the metal by the solution with the simultaneous formation of the coating as an integral part of the underlying metal. It is also common practice to employ solutions which result in the formation of oxides oi the metal treated. With these and similar processes, it is customary, after a coating of the desired nature is formed, to remove'the coated article from the solution, rinse, and dry it. Then, if desired, a paint finish can be applied at any convenient time.

The invention has as the object the treatment of metal surfaces for the prevention of corrosion and to provide a paint base for applying a film of corrosion-retarding chemicals and drying it on the metal surface. One specific object is to include a metal having a valence of two or more in a hexavalent chromium solution and thereby increase the corrosion-resistance of a coatin formed by drying a film of such a solution. Another specific object is to so select the chemicals employed that advantage may be taken of the corrosion-inhibiting quality of hexavalent chro mium compounds without excessive detrimental efiect from the usual hygroscopic nature of chromic acid. Another specific object is to increase the wetting ability of the solutions employed, thereby decreasing the necessity for a wetting agent.

These objects are accomplished by employing 40 a solution containing, in addition to a soluble phosphorous compound and a hexavalent chromiuin compound, a metal having a'valence of two or more. When the metal ls'introduced in the metallic form or is in a compound that will reduce some of the hexavalent chromium, the proportions must be such that hexavalent chromium will remain in the solution when it is used to coat the metal. It is usually more convenient to introduce the metal into the solution in the form of a soluble salt.

The solutions hereinafter described are effective on all industrial metals including iron, steel, zinc, cadmium, magnesium, tin, lead, nickel,

ment of ferrous surfaces, zinc, cadmium and nickel being especially important.

The effect of the presence in the solutions, of metals having a valence of two or more, is not entirely understood so far as the increase in corrosion-resistance is concerned. However, the fact that their presence provides a solution of 'a less hygroscopic nature than when, for example, only phosphoric and chromic acid is used, is more 10 easily explained. Chromic acid is well known for its tendency to absorb moisture. When metallic ions and ions of hexavalent chromium are introduced into a solution, an equilibrium is reached resulting usually in some dichromate of the metal.

15 Upon drying, the metal dichromate will not show the hygroscopic nature characteristic of chromic acid. When a metal having a valence of two or more is employed in the solution in the manner described, not only will the film on the surface of 20 the metal, resulting from application of the solution be less hygroscopic, but its corrosion-resistance is increased.

While the metals may be added in various other ways, it is preferable with most metals to intro- 25 duce themas phosphates or dichromates since this maintains the solution in its simplest form and no extra operations are necessary in the manufacture of the material.

A film of solution is applied to the metal surface and is dried thereon without the application of heat, or by baking. A small amount of free acid left in the coating does no harm from a practical viewpoint, if the surface is dried so that the treated article can be handled properly.

Solutions such as described below, containing chromic acid or dichromates, have some tendency to creep away from edges and perforations. These points are where corrosion usually starts and it is especially important that such areas retain the applied solution. In order to provide proper wetting, especially in cases where the cold solution is applied to cold metal, it is sometimes best to incorporate in the solution a suitable wetting agent. Amounts in the neighborhood of .5% have been found satisfactory under some conditions.

Where a heated solution is used, or the metal to be terated is heated, or both the solution and metal are employed at an elevated temperature, little or no wetting agent is required.

Following are examples of aqueous solutions which are especially adapted to the treatment of large structural members such as used in aluminum, copper, and alloys thereof, the treatbridges, freight cars, petroleum storage tanks and 3 so forth, where air drying of the applied film is especially. desirable.

Per cent 1. Zinc dihydrogen phosphate 2.8 75% phosphoric acid 2.5 Chromic acid 1.0

2. Chromic dihydrogen phosphate 2.5 75% phosphoric acid 2.5 Chromic acid 1.0

3. 75% phosphoric acid 3.0 Zinc dichromate 1.4

Usually it is most convenient to apply these solutions to structural members, etc., without heat, and .5% of a wetting agent may be added.

Following are'examples of solutions which after application will usually require forced drying such as baking because of the higher concentrations of the chemicals used.

8. Ammonium dichromate 10.0 Zinc dihydrogen phosphate 5.5

In the preceding examples, a wetting agent commercially known as Duponol WA (registered trade-mark) paste may be used. Its active base is sodium lauryl sulphate. Other compatible wetting agents may be employed, one precaution being that they must not be excessively oxidized by the hexavalent chromium. Other materials which have been found satisfactory as wetting agents and are to a more or less degree compatible with the solution are the sulphonation products of the higher alcohols.

Where solutions such as described above are properly dried on ferrous surfaces, there is some reaction resulting in ferric iron in the dried coating. Magnesium, cadmium, nickel and other metals having a valence of two or more, can be substituted for the metals given in the examples, and two or more of these metals may be employed in the same solution if desired.

The solution may be applied in any desired .manner such as dipping, spraying, brushing, and

so forth, care being taken to produce on the metallic surface, a uniform film. In every case the success of the invention lies in drying the film on the surface of the metal since the beneficial effects of the invention are obtained from the film dried on the surface.

It should be understood that in place of orthophosphoric acid or its compounds, other compounds of phosphorous, such as meta, pyIO, and hypo, may be employed for making up the soution, but in'a solution of the kind described such compounds will result in orthophosphoric compounds.

While a number of metals have been mentioned which added to the solution will attain the objects of the invention, zinc is preferred.

It is to be understood that the aforementioned materials, conditions, temperatures and other factors may be varied without departing from the spirit of the invention, the scope of which is to be measured only by the following claims.

What we claimis:

l. A process which comprises applying to an industrial metallic surface a film of an acidulous aqueous solution the chief compounds in the solution as applied containing the P04 radical, hexavalent chromium and a metal having a valence of at least two and acting as a cation, said film containing sufflcient chemicals to form a corrosion-resistant paint-holding coat when dried, and drying the film on the metallic surface.

2. A process which comprises applying to an industrial metallic surface a solution containing as its chief ingredients when applied phosphoric acid, chromic acid and a salt of a metal having a valence of at least two and acting as a cation. said film containing sufficient chemicals to form a corrosion-resistant paint-holding coat when dried and drying the film on the metallic surface.

3. A process which comprises applying to an industrial metallic surface a film of an acidulous aqueous solution the chief compounds in the solution containing the P04 radical, hexavalent chromium and zinc, said film containing sufilcient chemicals to form a corrosion-resistant paintholding coat when dried, and drying the film on the metallic surface.

4. A process which comprises applying to a surface of iron or steel a film of an acidulous aqueous solution the chief compounds in the solution as applied containing the P04 radical, hexavalent chromium and. a metal having a valence of at least two and acting as a cation, said film containing sufficient chemicals to form 'a corrosion-resistant paint-holding coat when dried, and drying the film on the metallic surface.

5. A process which comprises applying to a surface of iron or steel a film of an aqueous solution containing as its chief ingredients phosphoric acid, chromic acid and a compound of zinc, said film containing sufficient chemicals to form a corrosion-resistant paint-holding coat when dried, and drying the film of the solution on the metallic surface.

6. A process which comprises applying to a surface of zinc a film of an aqueous solution containing as its chief ingredients phosphoric acid, chromic acid and a compound of zinc, said film containing sufficient chemicals to forma corrosion-resistant paint-holding coat when dried, and drying the film of the solution on the metallic surface.

7. A process which comprises applying-to a surface of cadmium a film of an aqueous solution containing as its chief ingredients phosphoric acid, chromic acid and a compound of zinc, said film containing sufficient chemicals to form a corrosion-resistant paint-holding coat when dried, and drying the film of the solution on the metallic surface. v

8. An article of manufacture having a surface of an industrial metal covered with a corrosionresistant paint-holding coat having the composition resulting from drying on said surface a film of an acidulous aqueous solution the chief compounds in which solution as applied contain the P04 radical, hexavalent chromium and a metal having a valence of at least two and acting as a 5i of iron or steel covered with a corrosion-resistant paint-holding coat having' the composition resulting from drying on said surface a, film of an acidulous aqueous solution the chief compounds in which solution as applied contain the P04 radical, hexavalent chromium and zinc.

11. An article of manufacture having a surface of zinc covered with a corrosion-resistant paintholding coat having the composition resulting from drying on said surface a, film of an acidulous aqueous solution the chief compounds in which solution asapplied contain the P04 radical, hexavalent chromium and a metal having a valence of at least two and acting as a cation.

12. An article of manufacture having a surface of cadmium covered with a corrosion-resistant paint-holding coat having the composition resulting from drying on said surface a film of an acidulous aqueous-solution the chief compound in which solution as applied contain the P04 radical, hexavalent chromium and a metal having a valence of at least two and acting as a cation.

' JOHN S. THOMPSON.

ROBERT E. TANNER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,923,502 Prier n Aug. 22, 1933 2,030,601 McDonald Feb. 1.1, 1936 1,826,866 Cole Oct. 13, 1931 1,791,715 Darsey Feb. 10, 1931 1,719,464 Cole Nov. 19, 1929 

